Liquefied atmospheric gases, including nitrogen, oxygen and argon, are finding increasing uses in industry. Such liquefied atmospheric gases provide cryogenic capabilities for various industrial processes, are more economical to transport in merchant supply and provide ready and economical sources of gaseous product from liquid storage facilities. For instance, liquid nitrogen is increasingly used to freeze food products, to cryogenically embrittle used materials for cleaning or recycle and as a supply of gaseous nitrogen inerting medium for various industrial processes.
The cost of liquefied atmospheric gases is generally a factor in comparing the use of gaseous product and liquid product. It is apparent that additional energy or power to produce the necessary refrigeration to derive liquid products from air makes a liquid generating process more energy intensive than the typical gaseous product process. Therefore, to meet the increasing needs for liquid product in the area of atmospheric gases, it is desirable to have a process which is energy efficient in operation and economical from a capital cost factor. The prior art has frequently suffered from either or both of these aspects of producing liquefied atmospheric gases.
For instance, in U.S. Pat. No. 3,605,422, an air separation process is described wherein liquid oxygen and liquid nitrogen are derived directly from the distillation column 22. All of the feed air to the process enters the high pressure stage of that distillation column. The process also utilizes a gaseous nitrogen recycle refrigeration system including external refrigeration to provide sufficient cryogenic temperatures to produce the liquid product. Accordingly, this process is capital intensive.
British Patent No. 1,472,402 discloses a cryogenic air separation cycle wherein gaseous nitrogen is removed from a column, is liquefied in a separate system and the liquid is recovered in part as product and a part as reflux for the column.
In U.S. Pat. No. 4,152,130, a process is set forth for the production of liquid nitrogen and liquid oxygen from a cryogenic distillative separation of air using a two stage column and an air recycle refrigeration package. All of the feed air which is not recycled is delivered to the distillation column as feed, which includes both gaseous and liquefied air. The liquid atmospheric gas products are derived directly from the column specifically as liquid nitrogen from the reboil/condenser of the high pressure stage of the column and liquid oxygen from the sump of the low pressure stage of the column. Accordingly, the removal of liquid nitrogen gas products directly from the column effects the quantity of reflux available to operate the rectifications of the stages of the column, and increases in total air processed will be required accordingly.
In U.S. Pat. No. 4,375,367, an improved process derived from the previously discussed patent is set forth which requires less capital expenditure due to the reduction in utilization of compressor expander apparatus for the production of refrigeration. A freon refrigeration package is utilized in replacement for a tandem compressor and expander to reduce the capital costs with attendant increase in energy requirements. All of the liquefied air is fed directly to the distillation column, and a portion of the expanded and vaporized air is also fed to the column while a recycle stream providing refrigeration is returned to the feed air. Liquid nitrogen and liquid oxygen are derived directly from the distillation column and again effect the amount of reflux available to perform the rectification in the distillation column with the attendant increase in air necessary to be processed for a given quantity of liquid products and successful rectification.
In U.S. Pat. No. 4,400,188, a cryogenic system for the recovery of gaseous nitrogen is disclosed. A gaseous nitrogen recycle is utilized in which nitrogen is condensed at the base of a column rather than at an intermediate level and the resulting liquid is entirely utilized as internal reflux.
In U.S. Pat. No. 4,464,188, air and a nitrogen recycle are used to reboil a distillation column and the resulting liquid nitrogen and liquid air are both fed entirely to the column as reflux and feed respectively. Gaseous nitrogen is produced by the process.
Various problems of capital intensity and energy demands in cryogenic distillative separations for the production of liquid atmospheric gases are overcome by the present invention by the design and methods set forth below.
BRIEF SUMMARY OF THE INVENTION
The present invention is a process for the cryogenic distillative separation of air by fractionation in a distillation column to produce at least one liquid product stream selected from the group consisting of liquid nitrogen, liquid oxygen and/or liquid argon, wherein the improvement comprises cooling a feed air stream by appropriate refrigeration to produce at least a portion of the feed air stream as a liquid air stream; condensing preferably a product stream against at least a portion of the liquid air stream by indirect heat exchange while vaporizing the liquid air stream to produce a substantially gaseous air stream and a liquid product stream; and rewarming at least a portion of the gaseous air stream by indirect heat exchange with process streams, compressing said rewarmed gaseous air stream and recycling said compressed gaseous air stream to the feed air stream. Alternatively, a reflux stream can be condensed against the liquid air instead of a product stream; but this is less preferred. At least a portion of the substantially gaseous air stream can be fed without recycle for additional processing in a separatory operation to make additional product, alternative to fully recycling it to feed air.
In a detailed preferred embodiment, the present invention is a process for the cryogenic distillative separation of air to produce at least a liquid nitrogen product comprising the steps of compressing feed air to an elevated pressure and removing water, carbon dioxide and condensibles from the feed air, further compressing the feed air, splitting the feed air into a first split feed stream and a second split feed stream, compressing each of the split streams, cooling each split feed stream to a lower temperature by indirect heat exchange against process streams, expanding a first portion of the first split feed stream through a warm expander and recycling at least a part of the expanded stream to the feed air while providing refrigeration to the feed air by indirect heat exchange, expanding the second split feed stream through a cold expander and using at least a first portion of the expanded stream for a distillation step, recycling a second portion of the expanded second split stream to the feed air while providing refrigeration to the feed air by indirect heat exchange, removing an oxygen-enriched stream from the base of the distillation column, removing a gaseous nitrogen stream from the distillation column and condensing a first portion of the gaseous nitrogen stream against a process stream, condensing a second portion of the gaseous nitrogen stream from the distillation column against a second portion of the first split stream by indirect heat exchange to produce a liquid nitrogen product and recycling the second portion of the first split stream to the feed air.
Preferably, the distillation column is a two stage column with a high pressure stage and a low pressure stage.
Preferably, the first portion of the second split stream is introduced into the high pressure stage of the distillation column as feed to such column.
Preferably, a second part of the expanded first portion of the first split stream is introduced into the low pressure stage of the distillation column.
Preferably, the liquid nitrogen is subcooled against a part of the second portion of the first split stream and against a vapor portion of the liquid nitrogen product produced after said subcooled product is reduced in pressure and phase separated into a subcooled liquid nitrogen product and a nitrogen vapor phase stream. Alternatively, another part of the second portion of the first split stream is reboiled against condensing nitrogen gas from the high pressure stage of the distillation column in a side boiler/condenser with the resulting reboil stream recycled to feed air and the condensed nitrogen split into nitrogen reflux for the low pressure stage and liquid nitrogen product.
Alternatively, the oxygen-enriched liquid from the base of the low pressure column, which is indirectly heat exchanged with the first portion of the gaseous nitrogen stream, is removed in part to a side-arm column and is distilled by boiling liquid With a vaporized part of the second portion of the first split stream to provide a liquid oxygen product, a gaseous oxygen enriched stream and a condensed feed to the low pressure stage of the two stage distillation column.
Alternatively, the present invention is a process for the cryogenic distillative separation of air to produce at least a liquid nitrogen product comprising the steps of compressing feed air to an elevated pressure and removing water, carbon dioxide and condensibles from the feed air, splitting the feed air into a first split feed stream and a second split feed stream, cooling each split feed stream to a lower temperature by indirect heat exchange against process streams, expanding a first portion of the first split feed stream through a warm expander and recycling a part of the expanded stream to feed air while providing refrigeration to the feed air by indirect heat exchange, expanding the second split feed stream through a cold expander and introducing a first portion of the expanded stream into a reboiler/condenser in the base of a distillation Column to reboil the column and at least partially condense said expanded first portion, cooling, expanding and phase separating a second portion of the first split stream and combining the vapor phase of said second portion with said first portion of the expanded stream before its introduction into the reboiler/condenser, combining the liquid phase of the second portion of the first split stream with the at least partially condensed expanded first portion stream from said reboiler/condenser and phase separating the combined stream into a vapor phase feed stream and a liquid phase feed stream, introducing the vapor phase feed stream into the distillation column for rectification and removing a gaseous nitrogen stream from the top of the distillation column, condensing a portion of the gaseous nitrogen against oxygen-enriched liquid from the base of the distillation column and condensing another portion of the gaseous nitrogen stream against the liquid phase feed stream by indirect heat exchange in a side column to produce a liquid nitrogen product and a gaseous recycle stream to the feed air.